Data storage devices, such as disk drives, may allow host computers to store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive may include a plurality of magnetic recording disks, which may be mounted to a rotatable hub of a spindle motor and rotated at a high speed. An array of read/write heads may be disposed adjacent to data storage surfaces of the disks to transfer data between the disks and a host computer. The heads can be radially positioned over the disks by a rotary actuator and a closed loop servo system, and can fly in close proximity to the surfaces of the disks upon air bearings. The heads each typically contain a separate read element and write element.
Higher data storage density on the disks may be obtained by reading and writing data on narrower tracks on the disks, and/or by maintaining narrower flying height gaps between the heads and the data storage surfaces. The flying height of a head, i.e., the distance or spacing between the head and a surface of the adjacent disk, can vary in response to environmental conditions, such as temperature and/or pressure variations in the disk drive, and/or in response to head temperature variations. These changes can affect the distance that the tip of the head protrudes therefrom (i.e., pole tip protrusion). Maintaining the head flying height within a desired or acceptable range may become increasingly more difficult as that range is reduced to obtain higher data storage densities. Operation outside the acceptable range may result in an unacceptable read/write bit error rate and/or undesirable contact between a head and a data storage surface, and thus, potential loss of data and/or damage to the data storage surface.
Accordingly, some disk drives may controllably heat the head using a heater element to vary the flying height of the head. More particularly, dynamically controlled fly height or Fly Height Adjust (FHA) may be achieved using a heater element, such as an actuation coil, built into the head. When power is applied to the coil, the FHA head may protrude towards the disk. As such, by adjusting the power applied to the heater element, a fly height between the head and the disk surface can be maintained in a variety of changing environmental conditions, for example, due to changes in temperature, barometric pressure, etc.
For FHA heads, fly height may be adjusted in an open loop manner (i.e., without a feedback signal from the head) within a predetermined range of actuation. The amount of power compensation applied to the heater element may be determined based on a model or expected relationship between fly height and temperature and/or pressure. As such, temperature and/or pressure sensors may be required to determine the amount of power compensation for fly height adjustment. In addition, the accuracy of the compensation may be limited by the accuracy of the model. As such, variations in the actual heads from those of the model may lead to erroneous compensation.
For non-FHA heads, fly height may also be maintained in an open loop manner, for example, based on the air bearing surface (ABS) of the head. The ABS is used to build pressure to lift the head from the disk surface during normal drive operation, and is designed to keep the fly height within a certain range over varying environmental conditions to provide acceptable magnetic performance. However, the actual fly height may vary within the range based on the environmental conditions, which may thereby affect read and/or write performance. Tighter ranges may provide improved performance, but may lead to increased component costs. Moreover, if fly height is not within the desired range, non-FHA heads may be discarded, which may increase production costs.